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United States Patent |
5,648,438
|
Henry
,   et al.
|
July 15, 1997
|
Process for producing polymers with multimodal molecular weight
distributions
Abstract
The invention provides a continuous process for catalytically polymerizing
a monomer feed of ethylene and at least one comonomer which comprises
introducing a catalyst including a bulky ancillary ligand transition metal
compound and monomer feed in an upstream reaction zone for polymerization,
introducing further catalyst in a downstream reaction zone for further
polymerization, the peak temperature in the downstream reaction zone being
at least 50.degree. C. higher than in the upstream zone and being above
150.degree. C. The process economically produces polymer of good
properties which is melt processable.
Inventors:
|
Henry; David T. (Baton Rouge, LA);
McLain; Doulgas J. (Walker, LA);
Domine; Joseph D. (Humble, TX);
Mehta; Aspy Keki (Humble, TX);
Zafian; William Joseph (Pasadena, TX);
Baron; Norbert (Cologne, DE);
Folie; Bernard J. (Rhode-St. Genese, BE)
|
Assignee:
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Exxon Chemical Patents, Inc. (Wilmington, DE)
|
Appl. No.:
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221915 |
Filed:
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April 1, 1994 |
Current U.S. Class: |
526/65; 526/86 |
Intern'l Class: |
C08F 002/14 |
Field of Search: |
526/65,86,64
|
References Cited
U.S. Patent Documents
3536693 | Oct., 1970 | Schrader et al. | 260/94.
|
3575950 | Apr., 1971 | Gleason et al. | 260/94.
|
3756996 | Sep., 1973 | Pugh et al. | 260/87.
|
3917577 | Nov., 1975 | Trieschmann et al. | 26/94.
|
4085266 | Apr., 1978 | Nakai et al. | 526/65.
|
4665208 | May., 1987 | Welborn et al. | 526/124.
|
4937299 | Jun., 1990 | Ewen et al. | 526/119.
|
4939217 | Jul., 1990 | Stricklen | 526/114.
|
5055438 | Oct., 1991 | Canich | 526/141.
|
5064802 | Nov., 1991 | Stevens et al. | 526/134.
|
5096867 | Mar., 1992 | Canich | 526/160.
|
5169913 | Dec., 1992 | Staffin et al. | 526/65.
|
Foreign Patent Documents |
096221A2 | Dec., 1983 | EP.
| |
0129368B1 | Dec., 1984 | EP.
| |
0128046B1 | Dec., 1984 | EP.
| |
0128045B1 | Dec., 1984 | EP.
| |
0260999A1 | Mar., 1988 | EP.
| |
0260130A1 | Mar., 1988 | EP.
| |
0277004A1 | Aug., 1988 | EP.
| |
0277003A1 | Aug., 1988 | EP.
| |
0420436A1 | Apr., 1991 | EP.
| |
0533452A1 | Mar., 1993 | EP.
| |
278476A3 | May., 1990 | DE.
| |
1208120 | Oct., 1970 | GB.
| |
1251103 | Oct., 1971 | GB.
| |
1314084 | Apr., 1973 | GB.
| |
1332859 | Oct., 1973 | GB.
| |
WO90/03414 | Apr., 1990 | WO.
| |
WO91/04257 | Apr., 1991 | WO.
| |
WO92/00333 | Jan., 1992 | WO.
| |
WO92/02803 | Feb., 1992 | WO.
| |
WO92/14766 | Sep., 1992 | WO.
| |
WO92/15619 | Sep., 1992 | WO.
| |
WO93/03093 | Feb., 1993 | WO.
| |
WO93/08199 | Apr., 1993 | WO.
| |
WO93/08221 | Apr., 1993 | WO.
| |
WO93/13143 | Jul., 1993 | WO.
| |
Other References
Vickroy, et al., "The Separation of SEC Curves of HDPE into Flory
Distributions", Journal of Applied Polymer Science, vol. 50, 551-554
(1993).
|
Primary Examiner: Weber; Thomas R.
Claims
We claim:
1. A continuous process, in a single reactor, for catalytically
polymerizing monomer feed of at least two olefins having Ziegler-Natta
polymerizable bonds which comprises introducing a catalyst which includes
a metallocene and monomer feed composition into an upstream reaction zone
for polymerization, introducing further catalyst which includes a
metallocene into a downstream reaction zone for further polymerization,
the peak temperature in the downstream reaction zone being at least
10.degree. C. higher than in the upstream zone, and collecting polymer
having at least two fractions, each of which constitute at least 5 wt % of
the polymer, of different weight average molecular weight (M.sub.w), with
the M.sub.w of the fraction having the greater M.sub.w being at least 50%
greater than the M.sub.w of the fraction having the lesser M.sub.w, said
polymer having an overall M.sub.w /M.sub.n of at least 2.5 and a uniform
composition distribution.
2. The process according to claim 1 in which the polymerization occurs at a
pressure of at least 100 bar and in the presence of less than 30 wt % of
the feed composition as inert diluent.
3. The process according to claim 2, in which the polymerization occurs at
a pressure of at least 500 bar and in the presence of less than 30 wt % of
the feed composition as inert diluent.
4. The process according to claim 1 in which less than 50 wt % of the feed
composition is consumed in the polymerization reaction.
5. The process according to claim 3 in which less than 33 wt % of the feed
composition is consumed in the polymerization reaction.
6. The process according to claim 2 in which less than 50 wt % of the feed
composition is consumed in the polymerization reaction.
7. The process according to claim 6 in which less than 30 wt % of the feed
composition is consumed in polymerization reaction.
8. The process according to claim 2 in which the zones are spaces between
separating members in an autoclave with the baffles attached to an
autoclave agitator shaft, to provide a substantially back-mixed
environment within each zone and additional feed gas is supplied to such
spaces.
9. The process according to claim 4 in which the zones are spaces between
separating members in an autoclave with the baffles attached to an
autoclave agitator shaft, to provide a substantially back-mixed
environment within each zone and additional monomer feed is supplied to
such spaces.
10. The process according to claim 8 in which the upstream zone and the
downstream zone are separated by at least one zone to which substantially
no additional catalyst is injected.
11. The process according to claim 10 in which the monomer feed has an
inlet temperature of less than 65.degree. C. and catalyst is added in the
feed at an additional, initial zone prior to the upstream zone, so as to
produce, in the additional zone, less than 5% by weight of the final
polymer with an attendant temperature rise, in the initial zone, of less
than 50.degree. C.
12. The process according to claim 11 in which the monomer feed has an
inlet temperature of less than 65.degree. C. and catalyst is added in the
feed at an additional, initial zone prior to the upstream zone, so as to
produce in the additional zone, less than 2% by weight of the final
polymer with an attendant temperature rise, in the initial zone, of less
than 30.degree. C.
13. A continuous process for catalytically polymerizing monomer feed of at
least two olefins having Ziegler-Natta polymerizable bonds which comprises
introducing catalyst including a metallocene and the monomer feed in an
initial reaction zone for polymerization, introducing further catalyst
including a metallocene in a reaction zone downstream of the initial
polymerization for further polymerization in which the monomer feed has a
temperature of less than 65.degree. C. before its introduction and
catalyst is added for polymerization in the initial zone to produce less
than 5% by weight of the polymer with an attendant temperature rise of
less than 50.degree. C.
14. The process according to claim 3 in which the polymer has a melt index
of from 0.1 to 100 and a MWD of from 3 to 12.
15. A process comprising melt extruding polymer obtained by a process
according to claim 14.
16. A continuous process for catalytically polymerizing a monomer feed of
ethylene and at least one comonomer which comprises introducing a catalyst
including a metallocene and monomer feed in an upstream reaction zone for
polymerization, introducing further catalyst including a metallocene in a
downstream reaction zone for further polymerization, the peak temperature
in the downstream reaction zone being at least 50.degree. higher than in
the upstream zone and being above 100.degree. C.
17. A process for producing polymer comprising:
a) introducing monomer feed composition comprising at least two different
olefins into a polymerization reactor;
b) maintaining an upstream reaction zone and downstream reaction zone
within the reactor;
c) introducing a catalyst, which includes a metallocene, into the upstream
zone;
d) introducing a catalyst, which includes a metallocene, into the
downstream zone;
e) maintaining the downstream reaction zone at a temperature at least
100.degree. C. higher than the upstream zone; and
f) collecting resulting polymer.
18. A process for using a catalyst, which includes a metallocene,
comprising:
a) introducing monomer feed composition comprising at least two different
olefins into a polymerization reactor;
b) maintaining an upstream reaction zone and downstream reaction zone
within the reactor;
c) introducing a catalyst, which includes a metallocene, into the upstream
zone;
d) introducing a catalyst, which includes a, metallocene into the
downstream zone;
e) maintaining the downstream reaction zone at a temperature at least
100.degree. C. higher than the upstream zone; and
f) collecting resulting polymer.
19. The process of claim 1 wherein one olefin is ethylene.
20. The process of claim 3 wherein one olefin is ethylene.
21. The process according to claim 1 in which no hydrogen is added to the
reactor.
22. The process according to claim 1 in which less than 1 wt % hydrogen is
added to the reactor.
23. The process according to claim 13, in which the attendant temperature
rise is less than 30.degree. C.
24. The process according to claim 1, wherein the catalyst in the upstream
and downstream polymerization zones is the same.
Description
FIELD OF THE INVENTION
This invention relates to olefin polymerization processes and to polymers
made by such processes. The invention relates especially to processes and
low density (d<0.940) ethylene based polymers suitable for melt processing
e.g. extrusion.
BACKGROUND OF THE INVENTION
It is known to polymerize olefins (a) using free radical initiators, (b)
Ziegler-Natta catalysts based on titanium and vanadium transition metal
compounds and (c) using metallocene based catalysts. These main types are
associated with a certain type of polymer composition. Free radical
polymerization gives fairy broad molecular weight distribution and
extensive long chain branching. Titanium catalyst gives relatively narrow
molecular weight distribution products containing significant levels of
low molecular weight polymer having a high comonomer content. Finally
metallocene catalysts generally give very narrow molecular weight and
compositional distributions. Examples of the metallocene type are provided
in EP 260999; WO EP 92/02803 and WO 92/14766. Page 19 of WO 92/14766 shows
that in autoclaves generally a rise in the operating temperature takes
place from the top of the reactor to the bottom as a result of the heat of
polymerization. In cooled reactor systems such as tubular reactors that
rise can be reduced.
It has been thought desirable to extend the range of polymer compositions
producable by these metallocene and Ziegler-Natta catalyst systems and to
provide a greater choice of molecular weight and compositional
distribution.
Such attempts include blending different polymer compositions. EP 389611
(=WO 90/03414) uses narrow molecular weight distribution portions
polymerized with metallocene catalysts and blends them to achieve desired
molecular weight and compositional distributions. It is also known to
arrange reactors in series and establish different polymerization
conditions by varying temperature, monomer concentration and termination
agent (such as hydrogen). It is also known to introduce initiators into an
autoclave or tube at different positions or to use different catalyst
compositions or concentrations (U.S. Pat. No. 3,536,693; U.S. Pat. No.
3,575,950; GB 251103).
GB 1314084 describes a multi-zone polymerization with vanadium and titanium
based catalysts. The catalyst concentration exceeds 0.2.times.10.sup.-5
mol of transition metal per liter. Less than 30% of the monomer is
converted in the first zone per pass to give a higher molecular weight
material. Pressures are below 200 atmospheres. The monomer incorporation
in the different polymer fractions would vary depending on the selection
of catalysts in the first and second zone. In the examples generally the
second zone uses a titanium based catalyst in addition to a vanadium
catalyst. Titanium based catalyst tend to produce high molecular weight
materials. A higher temperature is used in the second zone; hydrogen has
to be added to limit the molecular weight. A broadened molecular weight
distribution product is obtainable but at a high catalyst cost, which
inclusion of a high comonomer, low molecular weight fraction and at low
productivity.
DD 278476 and GB 1208120 also use Ziegler-Natta catalyst.
Finally EP 128045, EP 260130 and WO 93/13143 use simultaneously introduced
different metallocene-based catalysts.
These prior art techniques for extending the range of available polymer
compositions have drawbacks ranging from high capital cost (series
reactors); deterioration in product quality (blending); difficulty of
predictably obtaining the desired target polymer compositions and
penalties in production rates of polymer.
It is amongst the aims of the invention to provide a relatively simple
process for predictably broadening molecular weight distribution at low
production cost and without producing a low molecular weight, high
comonomer fraction.
SUMMARY OF THE INVENTION
The invention provides a continuous process for catalytically polymerizing
a monomer feed composition comprising at least two olefins, preferably
with one being ethylene and at least one comonomer. The process comprises
introducing catalyst including bulky ancillary ligand transition metal
compound and monomer feed composition in an upstream reaction zone for
polymerization, introducing further catalyst in a downstream reaction zone
for further polymerization, the peak temperature in the downstream
reaction zone being at least 10.degree. C. higher than in the upstream
zone and separately a polymer having at least two fractions, each of which
constitute at least 5 wt % of the polymer, of different weight average
molecular weight (M.sub.w), the M.sub.w of the higher Mw fraction is at
least 50% greater than the Mw of the lower Mw fraction, said polymer
having a M.sub.w /M.sub.n overall ratio of at least 2.5. The molecular
weight of the fractions and their amount can be determined by
deconvoluting the information from GPC as explained in the Examples.
DETAILED DESCRIPTION
In addition to providing processes for use of olefin monomers and use of
catalysts, this invention provides a continuous process, in a single
reactor, for catalytically polymerizing monomer feed of at least two
olefins having Ziegler-Natta polymerizable bonds. The process comprises
introducing a catalyst, which includes a bulky ancillary ligand transition
metal compound, and monomer feed composition into an upstream reaction
zone for polymerization, introducing additional catalyst into a downstream
reaction zone to increase the peak temperature in the downstream reaction
zone at least 10.degree. C. higher than in the upstream zone, and
collecting polymer having at least two fractions, each of which constitute
at least 5 wt % of the polymer, of different weight average molecular
weight (M.sub.w), with the M.sub.w of the fraction having the greater
M.sub.w being at least 50% greater than the M.sub.w of the fraction having
the lesser M.sub.w, said polymer having overall M.sub.w /M.sub.n ratio at
least 2.5.
The term "Ziegler-Natta polymerizable bonds" is directed toward those
carbon--carbon bonds which are accessible to Ziegler-Natta catalysts. This
means that particularly carbon-carbon double bonds which are attached to
the first or last carbon atom, acetylenic bonds located in the same
position, and non-conjugated double bonds or triple bonds, and olefinic or
acetylenic bonds appearing in cyclic monomers.
Generally it is preferred, for this process described, that ethylene is
used as a monomer in the practice of this invention. By choice of
comonomer and amount of comonomer used and incorporated, the density of
the polymer product can be adjusted through the range from about 0.86
g/cm3 through about 0.96 g/cm3. Preferred upper ranges are about 0.94,
0.92 and 0.90. Preferred lower density ranges are about 0.89, 0.875 and
0.88.
The monomer feed composition, useful as described previously, may contain
less than about 50% inert diluent, but is preferred to have less than 30%.
The process may be run at pressures greater than about 100 bar, preferably
greater than about 500 bar. These polymerizations should be accomplished
with minimal addition of a chain termination agent, such as hydrogen.
Preferably less than 1 wt. % of hydrogen should be added. Ideally, none
will be added. At high pressures and temperatures, the presence of inert
diluent is less preferred, but may still have some applicability. With
high pressures and temperatures, the monomers in the feed composition
function as diluent but are not inert. This makes it likely that less than
50% of the monomers will be converted to polymers; for process flow
enhancement, it may be preferred that less than about 30% of monomer will
be polymerized. Monomer which is not polymerized may be separated and
recycled with proper equipment.
Maintaining separate temperature zones in this inventive process is
beneficial. Such separation helps provide the ability to make polymer of
differing molecular weights. That ability may be enhanced by providing
separating means, such as baffles, between zones to maintain homogeneity
within a single zone and heterogeneity among zones. Such intra-zone
homogeneity and inter-zone heterogeneity may be enhanced by interposing
zones into which no further (or minimal) catalyst is introduced between
the zones with catalyst introduction. A further enhancement of the effects
of this invention may be accomplished by introducing some catalyst
upstream of the first zone to produce, preferably, less than about 5 wt %
of total polymer before arrival in the "first" zone; ideally this would be
less than about 2 wt. %.
While ethylene is a useful comonomer, this invention may be practiced as
described here with any of at least two monomers having Ziegler-Natta
polymerizable bonds which are described elsewhere. Among the preferred
alpha-olefins are ethylene, propylene, butene, pentene, hexene, heptene,
octene, 4-methylpentene-1, and combinations thereof. More preferred
monomers include ethylene, propylene, butene, hexene, octene,
4-methylpentene-1, and their combinations.
As described, this inventive process, is useful for making polymers with
wide ranging molecular weight. Polymers may be made with melt indices
ranging from less than 1 to extremely high. Preferably, they will be
within the range of about 0.1 to about 100. Polydispersity (dispersion
index or M.sub.w /M.sub.n) may be obtained in the useful range of about
one to about 20. Higher ends of the preferred range will be about 15, 12,
8, and 5. Lower ends of this preferred range will be about 2, 2.5, 3, and
3.5. They will preferably be in the range of about 2.5 through about 12.
The inventive process described here is advantageously conducted in a
manner such that the reactor inlet temperature is less than 65.degree. C.
Useful polymers may be made when the temperature in the upstream zone is
less than about 50.degree. C. higher, preferably less than about
30.degree. C. higher than the reactor inlet temperature. It is beneficial
for the peak temperature in the downstream zone to be above the peak
temperature of the upstream zone. Preferably this difference, which may be
increased or decreased dependent upon catalyst sensitivity, will be at
least about 10.degree. C. higher than that of the upstream zone, more
likely, with less sensitive catalyst, that difference may need to be at
least about 50.degree. C. higher, or even being at least about 100.degree.
C. higher.
These various aspects, and combinations, of this invention as described
here provide polymers useful for many products. These polymers will be
particularly useful in making products for which extrusion, particularly
melt extrusion, processes are used including film blowing, injection
molding, blow molding protrusion, pultrusion, extrusion, extrusion
coating, and wire coating.
Such processes may be usefully applied to make films, sheets, profiles,
tubing, pipe, formed articles, and coated webs including paper.
With the term "upstream reaction zone" is meant a zone dose to, but
downstream of the polymerization reaction vessel inlet. The term
"downstream zone" refers to a reaction zone located downstream of the
upstream zone and closer towards the polymerization reaction vessel
outlet.
The combination of a split catalyst feed, a catalyst including a bulky
ancillary ligand transition metal compound and different operating
temperature in the zones provides a process which is both highly
productive and produces a polymer with desirable melt processing
characteristics. It is to be understood that within the broad concept,
variations may be employed such as feeding different catalysts into the
zone or introducing the same or different monomer feed in the downstream
zone.
The term "bulky ancillary ligand" refers to a ligand which is bonded to the
transition metal so as to be substantially stable under polymerization
conditions and which contains at least two atoms so as to sterically
restrict access to a catalytically active metal center.
The bulky ligand may contain a multiplicity of bonded atoms, preferably
carbon atoms, forming a group which may be cyclic with one or more
optional hetero atoms. The bulky ligand may be a cylopentadienyl
derivative which can be mono-or poly-nuclear. One or more bulky ligands
may be bonded to the transition metal atom. The transition metal atom is a
Group IV, V or VI transition metal of the Periodic Table of Elements.
Other ligands may be bonded to the transition metal, preferably detachable
by a cocatalyst such as a hydrocarbyl or halogen leaving group. The
catalyst maybe derived from a compound represented formula:
[L].sub.m M[X].sub.n
wherein L is the bulky ligand; X is the leaving group, M is the transition
metal and m and n are such that the total ligand valency corresponds to
the transition metal valency. Preferably, the catalyst is four co-ordinate
such that the compound is ionizable to a 1.sup.+ valency state. The bulky
ligand is sufficiently "bulky" to shield the metal atom The bond between
that ligand and the metal atom is normally resistant to the prevailing
reaction conditions relative to the leaving group and so that the ligand
is said to be "ancillary". The ligands L and X may be bridged to each
other and if two ligands L and/or X are present, they may be bridged. L
may be a cyclopentadienyl group. The resultant compound is referred to as
a metallocene. The metallocenes may be full-sandwich compounds having two
ligands L which are cyclopentadienyl group or half-sandwich compounds
having one ligand.
Certain types of metallocenes possess the necessary properties for the
process of this invention. For the purposes of this patent specification
the term "metallocene" is herein defined to contain one or more
cyclopentadienyl moieties in combination with a transition metal. The
metallocene catalyst component is represented by the general formula:
(C.sub.p).sub.m MR.sub.n R'.sub.p
wherein C.sub.p is a substituted or unsubstituted cyclopentadienyl ring; M
is a transition metal of group IV, V or VI of the Periodic Table of
Elements; R and R' are independently selected from halogen, hydrocarbyl
groups, or hydrocarboxyl groups having 1-20 carbon atoms; m=1 to 3, n=0-3,
p=0-3, and the sum of m+n+p equals the oxidation state of M. Various forms
of the catalyst system of the metallocene type may be used in the
polymerization process of this invention.
Exemplary of the development of these metallocene catalysts for the
polymerization of ethylene is found in the disclosure of U.S. Pat. No.
4,937,299 to Ewen, et al. and EP-A-0 129 368 published Jul. 26, 1989, both
of which have been fully incorporated herein by reference for US legal
purposes. These publications teach the structure of the metallocene
catalysts and includes alumoxane as the cocatalyst. There are a variety of
methods for preparing alumoxane of which one described in U.S. Pat. No.
4,665,208. Preferably, the alumoxane is methylalumoxane, especially MAO
having a degree of oligomerization of from 4 to 30 as determined by a
vapor phase osmometry determination of molecular weight and reacted
Al-content. Unreacted alkyl aluminum may be present generally in an amount
of less than 15 mol % as determined by titration of free alkyl aluminum.
Other cocatalysts may be used with metallocenes, such as trialkylaluminum
compounds or ionising ionic activators or compounds such as tri(n-butyl)
ammonium tetra (pentafluorphenyl) boron, which ionize the neutral
metallocene compound. Such ionising compounds may contain an active
proton, or some other cation associated with but not coordinated or only
loosely coordinated to the remaining ion of the ionising ionic compound.
Such compounds are described in EP A 0 277 003 and EP A 0 277 004 both
published Aug. 3, 1988 and are both herein fully incorporated by reference
for U.S. legal purposes. Further, the metallocene catalyst component can
be a monocyclopentadienyl heteroatom containing compound. This heteroatom
is activated by either an alumoxane or an ionic activator to form an
active polymerization catalyst system to produce polymers useful in this
present invention. These types of catalyst systems are described in, for
example, PCT International publications WO 92/00333 published Jan. 9,
1992, U.S. Pat. Nos. 5,096,867 and 5,055,438 EP A 0 420 436 and WO
91/04257 all of which are fully incorporated herein by reference for US
legal purposes. In addition, the metallocene catalysts useful in this
invention can include non-cyclopentadienyl catalyst components, or
ancillary ligands such as boroles or carbollides in combination with a
transition metal. Additionally, the catalysts and catalyst systems may be
those described in U.S. Pat. No. 5,064,802 and PCT publications WO
93/08221 and WO 93/08199 published Apr. 29, 1993, all of which are herein
incorporated by reference for US legal purposes.
The same or different activators may be used at various catalyst injections
points. The amounts injected can vary depending on the reactor contents so
as to maintain desired activator levels resulting from previous and
current activator injection.
As appropriate the catalyst and activator may be accompanied by compounds
which have a scavenging function and reduce the poison levels. Such
compounds include triethylaluminum, triisobutylaluminum, and hydrolysates
thereof.
As comonomer a C.sub.3 to C.sub.20 alpha mono-olefin may be used preferably
a mono-olefin having from 4 to 12 carbon atoms or copolymerizable
oligomers having a molecular weight above 30, preferably above 250. The
term olefin includes aromatics and saturated cyclic compounds such as
styrene. Preferably the comonomer is propylene, butene-1, pentene-1,
hexene-1, heptene-1, octene-1 or decene-1.
The monomer feed may further include a diluent which can be monomer which
does not react or inert materials such as ethane, butane or hydrocarbyl
materials which are liquid at room temperature and pressure; preferably
the diluent is less than 30 wt %, especially less than 10 wt % of the feed
stream overall.
The monomer feed may include minor amounts of termination agents, but, for
adiabatic process conditions, it is generally preferred that the molecular
weight should be determined by the local reaction temperature. Thus
preferably little (less than 1 wt %) or no termination agent (such as
hydrogen) is used for molecular weight control.
Preferably the polymerization is at a pressure of at least 100 bar
preferably at least 500 bar, in the substantial absence of added hydrogen
(preferably less than 1 wt %) and in the presence of less than 30 wt % of
the feed composition of an inert diluent. High pressure operation in the
substantial absence of hydrogen permits very high peak temperatures in the
downstream reaction zone so as to maximize yield, while giving a desired
low melt index product. The low temperature polymerization and the higher
temperature polymerization in the successive zones can be combined to
permit, at the same time, high monomer conversions and moderate melt
indices.
Suitably less than 50 wt % of the monomer feed is consumed in the
polymerization reaction, preferably less than 30 wt %. By running the
process so that there is a low monomer consumption and using unreacted
monomer to absorb the heat of the polymerization reaction, a product can
be obtained which has a relatively broad molecular weight distribution
combined with a relatively narrow variation in the incorporation of
comonomer in the high and low molecular weight fractions. There is no
fraction in which the low molecular weight fraction contains a
significantly higher level of comonomer.
Preferably the zones are spaces between separating members to define zones,
an example of which is baffles in a reactor, preferably an autoclave, but
other reactors, including tubular reactors may be used, preferably with
the baffles attached to an autoclave agitator shaft, to provide a
substantially back-mixed environment within each zone. The process of the
invention can then be performed in a single reactor without requiring
separate reactors arranged in series. Separating the upstream zone and the
downstream zone by at least one zone to which substantially no catalyst is
conveyed, helps to assure the bimodality which provides the greatest
rheological efficiency.
Each zone to which no catalyst is conveyed, may receive additional feed gas
so as to control the temperature increase and preferably maintain it at
the temperature of the preceding zone.
Further improvements in processability may to be achieved by a process in
which the feed has an inlet temperature of less than 65.degree. C.,
preferably less than 50.degree. C., especially less than 35.degree. C.
before its introduction. For producing a higher melt index material, a
higher inlet temperature can be practiced. Catalyst maybe added in the
feed in an additional, initial zone prior to the upstream zone, either
inside or outside of the polymerization reaction vessel. It may be an
additional upstream zone inside the reactor or may be located along the
path of the feed gas supply prior to entry into the vessel. It is
possible, and may be desirable, to add different catalyst at the different
points. This may be used to produce in the additional, initial zone less
than 5% by weight, preferably less than 2 wt %, of the final polymer with
an attendant temperature rise of less than 50.degree. C., preferably less
than 40.degree. C. or 30.degree. C. A low amount of relatively high
molecular weight material can thus be provided in a reactor blend with
only minor additional capital or product preparation costs.
This aspect of the invention may also be employed independently of the use
of different temperature zones in the reactor. Thus the invention secondly
provides a continuous process for catalytically polymerizing a monomer
feed of ethylene and at least one comonomer which comprises introducing a
catalyst including a bulky ancillary ligand transition metal compound and
monomer feed in an initial upstream reaction zone for polymerization,
introducing further catalyst in a reaction zone downstream of the initial
zone for further polymerization in which the monomer feed for the initial
zone has a temperature of less than 65.degree. C. and catalyst is added
for the first polymerization in the initial zone produce less than 5% by
weight of the polymer with an attendant temperature rise of less than
80.degree. C., preferably less than 50.degree. C.
Polymers obtained by the process of the invention have useful melt
processing characteristics and can be produced without significant
additional cost.
Suitably the polymer has a melt index of from 0.1 to 100. The level of
comonomer may be adjusted to give densities varying from about 0.86 to
0.95 g/cm.sup.3, preferably 0.87 to 0.94 g/cm.sup.3.
Most processes for converting polymer by extrusion into a shaped articles
such as tubes, wire coatings, cables, profiles, and films can be performed
efficiently with the polymer produced by the process of the invention.
The temperature differential between the polymerization zones may be,
depending on catalyst kinetics and final product requirements, from
50.degree. to 100.degree. C. Backmixing between the zones can be decreased
in different ways. One way of achieving this is to decrease the
cross-sectional flow area leading from one zone to another; another is by
increasing the separation. The operating temperature and conversion can be
increased also by using monomers which raise the molecular weight at a
given temperature such as polyenes so that even at higher reaction
temperature, a desired target melt index can be produced.
Using single autoclave reactor lay-outs, the desired polymerization of high
and low molecular weight portions can be completed within an overall
residence time of from 5 to 90 seconds. The overall average residence time
is determined by comparing reactor volume and throughput.
Using the aforementioned process and reactor layouts, polymer compositions
can be obtained which are highly melt processable having I.sub.21 /I.sub.2
ratios, referred to herein as MIR of above 15, preferably above 25.
Typical molecular weight distributions (M.sub.w /M.sub.n) are at least 2.5
and may range from 3 to 12, preferably from 4 to 8.
Superior product properties result from a relatively uniform composition
distribution. The compositional distribution can be determined down to a
minimum crystallinity level by suitable test methods. A useful method is
explained in detail in PCT patent application WO 93/03093 published Feb.
18, 1993.
The high MIR compositions permit easy melt processing, extrusion etc. into
profiles, tubing and wire coating without detracting from the main benefit
of narrow CD materials. The process permits very high outlet temperatures
for a product having a low, average MI, thereby increasing monomer
conversion and improving process economics.
A summary of possible conditions in the respective zones is set out in
table A. The table provides a mere example and general guidance. Precise
conditions will vary depending catalyst selection, comonomer, kinetics and
final product requirements.
TABLE A
______________________________________
Initial zone prior to
Upstream main
Down stream
upstream zone
zone zone
______________________________________
Inlet temperature
<50.degree. C.
<100.degree. C.
<150.degree. C.
(<75.degree. C.)
End temperature
<100.degree. C.
<150.degree. C.
>150.degree. C.
<300.degree. C.
(>180.degree. C.)
.DELTA. T 50.degree. C. approx.
50.degree. C. approx.
50.degree. C. approx.
(40.degree. C.)
MI; M.sub.w
MI < 0.1 M.sub.w > 100,000
M.sub.w > 20,000
M.sub.w > 200,000
Proportion of
<5% *10-90% *90-10%
total polymer %
(2%) (20-80%) (80-20%)
______________________________________
Overall characteristics
______________________________________
MI 0.1-100
MWD 3-12
(4-8)
Density 0.866-0.94
______________________________________
Preferred ranges are given in parentheses.
*Ignoring any material produced in the additional upstream zone.
The invention is illustrated by the Examples
The Reactor
The polymerization in the runs were carded out in a high pressure
continuous polymer production facility at the indicated pressures.
The autoclave reactor has an agitator with baffles which define a first
upstream zone (referred to herein as zone 1) and four downstream zones
referred to herein as zones. Each zone is slightly backmixed by the
agitator, but backmixing between the zones is limited by the baffles.
Separate catalyst injection points are provided at zone 1 and zone 2 and
optionally the other zones. Fifty percent of the reactor feed (monomers
and diluent, if used) was fed from the high pressure compressor to zone 1
of the reactor. The remainder of the reactor feed was fed to three zones
(1,2 and 4) through side feeds controllable by appropriate valves. Amounts
fed to zones 2 and 4 were just enough to prevent backflow from the reactor
and concomitant plugging of the inlet.
At the outlet of the reactor there is located a catalyst killer injection
point. The killer used in the examples was water which was added in a
small amount related to the catalyst injection rate so as to deactivate
the catalyst and prevent polymerization downstream and during recycling of
the monomer mixture as explained later on.
Downstream of the killer injection point is a let down valve which reduces
the pressure of the monomer/polymer mixture immediately prior to entry
into a high pressure separator (HPS) for separating the monomer/polymer
mixture. The polymer rich phase was taken from the HPS for further
processing; the monomer rich phase is recycled to the reactor via the high
pressure recycle system consisting of a series of coolers and a high
pressure compressor which supplies the feed to the polymerization reactor
The Process
Dimethylsilyl bis(tetrahydroindenyl) zirconium dichloride was used as a
transition metal catalyst component. Methyl alumoxane in toluene (30% by
weight) obtained from Ethyl Corp. was used as an activator. The Al:Zr mol
ratio was 1000. The catalyst components were combined in a solution of
Isopar.TM. C and supplied by a pump arrangement to two separate catalyst
injection points one in zone 1 and another in zone 2 for these
non-limiting examples.
Catalyst feed rates was adjusted to maintain the indicated zone
temperatures.
EXAMPLE 1
Table 1 sets out the polymerization conditions for an ethylene propylene
mixture.
TABLE 1
______________________________________
Main feed Side feed Zone 1 reactor
temperature temperature temperature
______________________________________
Run 1 (80.degree. F.)
(75.degree. F.)
(210.degree. F.)
27.degree. C.
24.degree. C.
99.degree. C.
Run 2 (70.degree. F.)
(60.degree. F.)
(195.degree. F.)
21.degree. c.
15.5.degree. C.
90.5.degree. C.
Run 3 (65.degree. F.)
(45.degree. F.)
(185.degree. F.)
18.degree. C.
7.2.degree. C.
85.degree. C.
______________________________________
Zone 2 reactor temperature
C.sup.3 = in feed mol %
______________________________________
Run 1 (380.degree. F.) 58
190.degree. C.
Run 2 (355.degree. F.) 66
179.degree. C.
Run 3 (340.degree. F.) 65
171.degree. C.
______________________________________
Table 2 sets out the analytical details obtained by GPC and other analysis
of the compositions produced in the runs.
TABLE 2
______________________________________
MI Density MIR MWD
______________________________________
Run 1 13 0.899 29 4.6
Run 2 17 0.882 31 4.6
Run 3 12 0.882 28 4.6
______________________________________
Weight Average Molecular Weights
(and weight percentage)
Overall First Second
Third
UC fraction fraction
fraction
______________________________________
Run 1 51900 121000 47800 21700
(20%) (44%) (30%)
Run 2 52300 121000 46700 19100
(22%) (43%) (31%)
Run 3 56700 139000 50500 22800
(19%) (46%) (29%)
______________________________________
To probe the molecular make-up of the molecular weight distributions of
product made by the process of this invention, further mathematical
deconvolutions of the GPC data were performed. The procedure involves
mathematically matching the GPC data of the product with a combination (at
different molecular weights) of standard single-site catalyst produced
components (i.e., each component having a 2.0 polydispersity index,
Flory-type distribution). The results of this treatment are in Table 2 for
the three inventive process produced products, where the amounts and
molecular weight details of the individual components are shown. (By way
of comparison, a standard metallocene produced polymer can typically be
matched similarly with a single such component.) This data analysis was
accomplished in a manner like that described by Vickroy, et al. in "The
Separation of SEC Curves of HDPE into Flory Distribution," Journal of
Applied Polymer Science, Vol 50, 551-554 (1993).
Gel Permeation Chromatography (GPC) is a liquid chromatographic technique
used to measure the molecular weight (MW) and molecular weight
distributions (MWD) of polymers.
______________________________________
Equipment:
______________________________________
Instruments Water model 150C
chromatograph
Columns Three (3) Shodex AT-80M
(mixed bed)
Solvent 1,2,4,trichlorobenzene
(HPLC grade)
Operating Temperature 145.degree. C.
Conditons Flow rate 1 ml/min
Run time 60 min
Injection vol. 300 microliters
______________________________________
Sample Preparation: Samples are prepared by weighing 10 mg of sample into a
20 cc vial. Ten (10) cc of trichlorobenzene (TCB) is added and the mixture
stirred at about 180 C till all the sample is dissolved (about 30 min).
The solutions are then transferred to auto-sampler vials and placed in the
150C GPC.
Calibration: The instrument was calibrated by using National Bureau of
Standards Polyethylene 1475, fitting the results to a third order
calibration curve.
Data Acquisition and Evaluation: Data are acquired and all calculations
performed using Waters "Expert-Ease" software running on a VAX 6410
computer.
EXAMPLE 2
This is a predictive Example; not based on an actual test. The predictive
model assumes no backmixing of monomers and/or heat. Its predictive
quality was tested and found to be fairly accurate on the basis of actual
tests such as those set out in Example 1.
The reactor conditions were as follows T=160.degree. C. (320.degree. F.),
the target MI of the overall polymer composition was 4.5 and the
percentage of butene-1 derived units in the polymer was 11% yielding an
approximate 0.90 density.
At an inlet temperature of 35.degree. C. (95.degree. F.) for the inlet end
of the reactor, one obtains a melt index of 0.0002 corresponding to an
M.sub.w of from 1.0.times.10.sup.6 to 0.60.times.10.sup.6. The molecular
weight is in practice lowered toward 0.6.times.10.sup.6 because of the
generation of hydrogen as a by-product of the polymerization and its
accumulation in a continuous high pressure reactor.
The amount of the high molecular weight material produced can be kept low
by injecting a suitable low level of catalyst and restricting the
residence time at the low temperature.
If appropriate, these small levels of high molecular weight polymer can be
generated in tubular feed stream conduits leading from the compressor to
the autoclave pressure vessel.
It is believed that because the amounts of high molecular weight product
are so small that materials containing them can be generated and processed
without undue difficulty
It is clear that this technique can be used so that this polymerization
precedes a main upstream polymerization under constant, backmixed
conditions leading to the absence of temperature variation in the reactor.
Alternatively it may be used in addition to a multi zone polymerization in
an autoclave as described in Example 1.
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